If you are simply looking for feedback, even before a game launches, you can use such services as Google Surveys, or even post to forums with a promo (ie. give feedback for a chance to win a free game!) These tend to work well with quick reaction feedback, ie. Do you like the name of the game? What type of games do you like? If which gameplay feature is more important to you? In these sort of surveys you want over 100 people (or more, depending on the margin of error you are comfortable with), and you want to factor in some sort of filter so you know the people that answer are relevant to you.
If you got a web based game you can use such sites like usertesting.com which provides live user testing. You can get recorded feedback along with a video of how the person went through the game experience. Other focus group testing sessions can cost you about $50 a person, at least that's about what it is around the bay area. Or you can also just ask a lot of friends to play and watch them play! It's really quite useful and I learned a lot about my own games, things that I thought were obvious ended up causing utterly game breaking experiences for others.
The other thing you should do is set up metrics tracking in your game. Such services like GameAnalytics does a good job. You can gather a lot of info about retention, what actions people are taking in game, and what's causing them to drop off. It doesn't quite reveal the details of what specifically cause each of these things, but metrics helps you narrow down your focus to look at the trouble spots in your game.
In the end, you need to do multiple things because each method tests different things, and it's that full scope that reveals what specifically you want to iterate on in your game. In other words, you always have a limited amount of time to build the next feature. You always want to be sure what you build addressing the most pressing need based on feedback and metrics. I know this sounds like a lot but you can do each thing in phases. You always want to keep in mind that with each update, you want some way to get feedback so that your next iteration is always in the right direction. Towards fun!